1. Field of the Invention
This invention relates to electronic ballasts for powering a high frequency electrodeless fluorescent lamp. An electric arc appears in the lamp connector when disconnecting the lamp under power. It is destructive to the ballast and dangerous to the personnel replacing lamps. Also, arcing may be caused by poor connections in the fluorescent lamp wiring or disconnecting of crimped wire from the connector and may create a fire hazard.
2. Description of the Related Art
Since the fluorescent lamp is powered from a current source with high operating frequency (250 kHz or more), such as a self oscillating DC to AC inverter, a stable arc path is established between connector pins or between the connector pin and a lamp wire, even if two separated portions are placed by a distance of up to 1-2 inches from each other. At high frequency, recombination time of particles (electrons and ions) in arc plasma becomes comparable with AC frequency. When crossing zero current, it is not enough time for particles to be recombined in gas molecules and to stop the current flow and cancel the arc. Therefore, it should be done artificially by stopping oscillation in the ballast inverter with a shut down circuit susceptible to the arc.
There are a few known drawbacks, however. When the arc appears in the connector, there is no actual change in ballast high frequency voltages and currents that could be used for arc detection, as the voltage drop across the arc is negligible in relation to lamp rated voltage. Additionally, there is a large increase in ballast voltages and currents during normal lamp starting and they are also effected by low frequency 100/120 Hz steady-state modulation caused by the AC line rectifier. To avoid false responses, the arc detection circuit should not be susceptible to all of these disturbances, which occur during normal lamp operation.
The prior art teaches arc cancellation in the lamp connector by mechanically interlocking the ballast inverter when unplugging the lamp. In some ICETRON/ENDURA electrodeless lamps, additional pins are used in the connector to disconnected some components of the ballast inverter without which oscillations in the inverter cannot exist. However, the required three-wire connector is thick, expensive, and not applicable for a lamp that is placed a distance from the ballast.
Other references disclose different sensing means for arc detection, but they are only associated with low frequency AC devices, like electrical welding equipment, and not applicable for an arc in high frequency ballasts. Furthermore, the purpose of such devices is arc stability. In contrast, the purpose of the present invention in a ballast is arc cancellation.
Therefore, a protection method and circuit with fast arc cancellation capability is still needed. Another feature of the protection circuit should be a reset capability for restarting a reconnected lamp. The protection circuit is also required in the event that the ballast is mistakenly turned on without a lamp.